1. Field of the Invention
The present invention relates to transporting fluids in channels, such as conduits. In particular, the present invention relates to a method and apparatus for transporting fluids in channels and reducing contamination and/or dilution of fluid being transported.
2. Description of the Related Art
Fluid handling, for example, liquid handling in systems such as analyzers (chemical, biological and immunological), and blood typing systems (e.g., the Ortho ProVue™ system manufactured by Ortho-Clinical Diagnostics, Inc.) is known in the art. In addition, fluid handling in microfluidic systems as described in U.S. Pat. Nos. 6,453,928 and 5,992,820 and in PCT publication Nos. WO 97/21090 and WO 02/18949 is also known in the art. Fluid handling systems that use air to separate different liquid samples, or to identify or provide information for different samples are also known in the art. See, e.g., U.S. Pat. Nos. 4,853,336, 4,259,291, 3,479,141, 2,797,149 and 2,879,141. See also, WO 88/04052.
In fluid handling systems, it is generally known to use one fluid, hereinafter referred to as a working fluid (water, saline, etc.) to better control the fluid that is being handled, such as being aspirated or dispensed, by hydraulically coupling the metering pump motion to the fluid being metered. The working fluid helps ensure that the fluid being transported will be moved in a manner that-mimics the motion of the metering pump. Air based systems or systems with part air and part working fluid are subject to the compressibility of the air; thus metering precision and accuracy may be degraded.
A disadvantage with systems filled with working fluid only is that the fluid in the system can either dilute the fluid being metered or interact chemically with that fluid. The mixing of these fluids can occur because of turbulence, diffusion at the interface, and residual boundary layer fluid on the internal walls. It is generally known to use air gaps to separate fluids being transported. The size of the air gap is generally minimized such that the increased compressibility associated with the air ideally is not so large that the handling precision and accuracy is degraded substantially. The air gap or bubble can perform the function of “scrubbing” the internal walls of residual fluid, along with providing physical separation between the two fluids.
Several factors can result in increased mixing between these two fluids, even in the presence of an air gap, which reduces the effectiveness of the air gap and can result in unsatisfactory commingling of the two fluids. Some of these factors are listed below:                Smoothness (conversely roughness) of the interior surface of the conduit where the fluid flows, since increased roughness will retain greater amounts of fluid.        Changes in inner diameter of the conduit, such as a lumen since a change in internal diameter will induce turbulence.        Surface wetability of the conduit surface.        Control of the working fluid at the end of a probe on aspiration.        Contact angle of the working fluid and fluid being transported to the channel surface.        Rheology of the fluids being transported since high viscosity fluids will increase the size of the boundary layer.        Accordingly, no air gap or even a single air gap between the working fluid and fluid being handled is unsatisfactory for many applications, including clinical chemistry diagnostics, immunodiagnostics, blood screening, immunohematology, and microfluidics, where the effect of contamination with the working fluid can be significant.        